Method for producing an alpha-alumina powder

ABSTRACT

A method for producing an α-alumina powder is provided. T method for producing an α-alumina powder comprising steps of: (1) pulverizing a metal compound having a full width at half maximum (Ho) of a main peak in XRD pattern to obtain a seed crystal having a full width at half maximum (H) of the main peak in XRD pattern in the presence of pulverizing agent, (2) mixing the obtained seed crystal with an aluminum salt, (3) calcining the mixture, and wherein a ratio of H/Ho is 1.06 or more.

FIELD OF THE INVENTION

The present invention relates to a method for producing an α-aluminapowder having a high α-ratio and large BET specific surface area.

DESCRIPTION OF RELATED ART

α-alumina is one of aluminum oxides, which is represented by formulaAl₂O₃ and has a corundum structure, and widely used as a raw materialfor producing a sintered body such as a translucent tube.

From the standpoint of improvement in the strength of a sintered body,α-alumina used as a raw material is required to have a high α-ratio andlarge BET specific surface area.

SUMMARY OF THE INVENTION

The present inventors have investigated a method for producing anα-alumina powder, resultantly leading to completion of the presentinvention.

Namely, the present invention provides a method for producing anα-alumina powder comprising steps of:

-   -   (1) pulverizing a metal compound having a full width at half        maximum (hereinafter abbreviated to “FWHM”)(Ho) of a main peak        in X-ray diffraction (hereinafter abbreviated to “XRD”) pattern        to obtain a seed crystal having FWHM (H) of the main peak in XRD        pattern in the presence of pulverizing agent,    -   (2) mixing the obtained seed crystal with an aluminum salt,    -   (3) calcining the mixture, and    -   wherein a ratio of H/Ho is 1.06 or more.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 shows a method to calculate FWHM Ho of a metal compound and FWHMH of a seed crystal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method for producing an α-alumina powder of the present inventioncomprises a step (1) of pulverizing a metal compound having a FWHM(Ho)of a main peak in XRD pattern to obtain a seed crystal having FWHM (H)of the main peak in XRD pattern.

The metal compound may advantageously be that promoting phasetransformation from an aluminum compound into α-alumina in calcinationdescribed later. Examples of the metal compounds include metal oxidessuch as α-alumina(Al₂O₃), α-iron oxide(Fe₂O₃) and α-chromiumoxide(Cr₂O₃); metal hydroxides such as diaspore (AlOOH), preferablymetal oxides, and further preferably α-alumina.

Pulverizing is conducted in the presence of pulverizing agent.

The pulverizing agent may advantageously be that promoting an efficiencyof pulverizing. Examples of the pulverizing agent include alcohols suchas ethanol, propanol; glycols such as ethylene glycol (molecularweight[mw]: 62, boiling point[bp]: 197.2° C.), polyethylene glycol,propylene glycol (mw: 76, bp: 187.4° C.) polypropylene glycol (mw: 134,bp: 231.8° C., in case of dipropylene glycol), 1,3-butylene glycol (mw:90, bp: 207.5° C.), 1,4-butylene glycol (mw: 90, bp: 235° C.); aminessuch as triethanol amine; fatty acids such as palmitic acid, stearicacid and oleic acid; metal alkoxides such as aluminum alkoxide; carbonmaterials such as carbon black and graphite, preferably glycols, morepreferably glycols having a MW of from 50 to 300 and boiling point offrom 150° C. to 300° C., further preferably polypropylene glycol. Thepulverizing agent may be used independently or two or more of them maybe used in combination. The amount of the pulverizing agent is usuallyabout 0.01 parts by weight or more, preferably about 0.5 parts by weightor more, further preferably about 0.75 parts by weight or more andusually about 10 parts by weight or less, preferably about 5 parts byweight or less, further preferably 2 parts by weight or less based on100 parts by weight of the metal compound.

When conducted in the presence of water, pulverizing may be conductedfurther in the presence of dispersant or surfactant. Examples of thedispersant include acids such as nitric acid, hydrochloric acid,sulfuric acid, acetic acid and oxalic acid; alcohols such as methanol,ethanol, isopropyl alcohol; aluminum salts such as aluminum nitrate,aluminum chloride, aluminum oxalate and aluminum acetate. Examples ofthe surfactant include anionic surfactants, cationic surfactants,nonionic surfactants and amphoteric surfactants.

Pulverizing may be conducted in batch-wise or continuous process.Pulverizing may advantageously be conducted, for example, by using apulverizer such as ball mill, vibration mill, planetary mill, pin mill,medium-agitating mill and jet mill. In pulverizing, it is preferable todecrease contamination, and for this, it is recommended to use alumina,preferably alumina having a purity of 99% by weight or more as thematerial of member, which is contacted with the aluminum compound, suchas pulverizing medium, vessel, nozzle and liner.

Further, the seed crystal obtained by pulverizing may be classified. Byclassification, 50% by weight or more, preferably 70% by weight or more,further preferably 90% by weight or more of coarse particles (forexample, particles with a particle diameter of about 1 μm or more) maybe removed from the seed crystal.

The seed crystal obtained in the above method has an average primaryparticle diameter of usually about 0.01 μm or more, preferably about0.05 μm or more, and usually about 0.5 μm or less. The seed crystal hasa BET specific surface area of usually about 12 m²/g or more, preferablyabout 15 m²/g or more, and usually about 150 m²/g or less.

Pulverizing is conducted under conditions which change a metal compoundhaving FWHM of Ho into a metal compound having FWHM of H, wherein theratio of H to Ho is about 1.06 or more, preferably about 1.08 or more,and usually about 5 or less, preferably about 4 or less, furtherpreferably about 3 or less. The ratio of H/Ho represents a degree ofpulverizing, and is calculated from FWHM(Ho) of a main peak between 45degrees and 70 degrees in XRD pattern measure before pulverizing andFWHM(H) of the main peak in XRD pattern measured after pulverizing asshown in FIG. 1.

When a metal compound is α-alumina and X-ray source is CuK α beam, theratio of H/Ho represents may be calculated from FWHM(Ho) of alumina(116)diffraction peak observed at 2θ of about 57.5 degree, in XRD patternbefore pulverizing, and FWHM(H) of the alumina(116) diffraction peak inXRD pattern after pulverizing.

Regarding α-iron oxide(Fe₂O₃), α-chromium oxide(Cr₂O₃) ordiaspore(AlOOH), a main peak thereof between 45 degrees and 70 degrees,which is usually a peak of (116), is observed at near position to thatof α-alumina in XRD pattern measured using CuKα beam as X-ray source.

The method of the present invention comprises further a step (2) ofmixing the obtained seed crystal with an aluminum salt.

The aluminum salt may be a compound converting into α-alumina bycalcination described later, and examples thereof include an aluminuminorganic salt such as aluminum nitrate, aluminum sulfate, aluminumammonium sulfate and ammonium aluminum carbonate hydroxide; and analuminum organic salt such as aluminum oxalate, aluminum acetate,aluminum stearate, aluminum lactate and aluminum laurate, and preferablyan aluminum inorganic salt, further preferably aluminum nitrate.

Mixing in step (2) may be conducted by using a ball mill, vessel withagitator and the like.

Mixing may be advantageously conducted, for example, by a method inwhich an aluminum salt is added to a solvent to obtain a solution orslurry, a seed crystal is added to this, and then, the solvent isremoved. By the above-mentioned mixing, an aluminum salt in which seedcrystals are uniformly dispersed is obtained.

In mixing, a seed crystal may be dispersed in a solvent before adding tothe above-mentioned solution or slurry, and then, the solvent isremoved.

Further, mixing may be conducted also by a method in which a seedcrystal is added to an aluminum salt, and the mixture is stirred.Stirring may be advantageously conducted by using an apparatus such as avertical granulator and Henschel mixer. In this mixing, a seed crystalmay be dispersed in a solvent before addition to the above-mentionedaluminum salt, and then, the solvent is removed.

The amount of the seed crystal is usually about 1 parts by weight ormore, preferably about 2 parts by weight or more, further preferablyabout 4 parts by weight or more and usually about 50 parts by weight orless, preferably about 40 parts by weight or less, further preferably 30parts by weight or more based on 100 parts by weight of the total amountof the seed crystal and the aluminum salt.

The obtained mixture may further be subjected to drying. Drying may beconducted at temperature of usually about 100° C. or less by usingfreeze dryer, vacuum dryer or the like.

The method of the present invention comprises further a step (3) ofcalcining the mixture obtained above.

The calcination may advantageously be conducted by using an apparatussuch as a tubular electric furnace, box-type electric furnace, tunnelfurnace, far-infrared furnace, microwave furnace, shaft furnace,reflection furnace, rotary furnace and Roller Hearth furnace. Thecalcination may be conducted in batch-wise or continuous. It may beconducted in static mode or flow mode.

The calcination temperature is not lower than the temperature at whichthe aluminum salt transforms to an α-alumina, usually 600° C. or higher,preferably about 700° C. or higher and usually about 1000° C. or lower,preferably about 950° C. or lower. The calcination time is usually 10minutes or more, preferably about 30 minutes or more and usually about24 hours or less, preferably about 10 hours or less.

The calcination is usually conducted under air or an inert gas such asN₂ and Ar. The calcination may also be conducted under air havingcontrolled partial pressure of water vapor, for example, air having apartial pressure of water vapor of 600 Pa or less.

The obtained α-alumina powder may be subjected to pulverizing. Thepulverizing may be conducted, for example, by using a medium pulverizersuch as a vibration mill and a ball mill, or an pneumatic pulverizersuch as a jet mill. Further, the α-alumina powder may be subjected toclassification.

An α-alumina powder obtained by the method of the present invention hasan average particle diameter of usually about 0.01 μm or more,preferably about 0.05 μm or more, and usually about 1 μm or less,preferably about 0.1 μm or less, α-ratio is about 90% or more,preferably about 95% or more and BET specific surface area of about 8m²/g or more, preferably about 13 m²/g or more, more preferably about 15m²/g or more and about 100 m²/g or less, preferably about 50 m²/g orless, more preferably about 30 m²/g or less.

The α-alumina powder has, as described above, a high α-ratio and largeBET specific surface area, therefore, this powder is useful as a rawmaterial for producing an α-alumina sintered body with high strength.The resulted α-alumina sintered body is suitable as a member for whichhigh strength is required such as a cutting tool, bioceramics,low-resistance routing pattern ceramics (for example, alumina ceramicswith copper patter thereon) and bulletproof board. The α-aluminasintered body is, due to chemical stability such as excellent corrosionresistance, used as a part of an apparatus for producing a semiconductorsuch as a wafer handler; an electronic part such as an oxygen sensor; atranslucent tube such as a sodium lamp and metal halide lamp; or aceramic filter. A ceramics filter is used for removal of solidcomponents contained in a exhaust gas, for filtration of aluminum melt,filtration of drinks such as beer, or selective permeation of a gasproduced at petroleum processing or CO, CO₂, N₂, O₂, H₂ gas. Theα-alumina powder may be used as a sintering agent for ceramics such asthermally conductive ceramics (for example, AlN), YAG and phosphors.

Further, the α-alumina powder may be used as an additive for toner orresin filler. for improving head cleaning property and frictionresistance by addition thereof to an application layer of a magneticmedium of application type. Also, the α-alumina powder may be used as anadditive for cosmetics or brake lining.

Furthermore, the α-alumina powder is used as a polishing material. Forexample, a slurry obtained by dispersing an α-alumina powder in a mediumsuch as water is suitable for polishing of semiconductor CMP andpolishing of a hard disk substrate. A polishing tape obtained by coatingan α-alumina particle on the surface of a tape is suitable for precisepolishing of a hard disk and magnetic head.

EXAMPLES

The present invention is described in more detail by following Examples,which should not be construed as a limitation upon the scope of thepresent invention. The properties of an α-alumina and a seed crystalwere evaluated as follows.

(1) α-ratio

It is calculated according to the following equation (i) using the peakstrength I_(25.6) at 2θ=25.6°, which is corresponding to a peakintensity of α-alumina (012) and the peak strength I₄₆ at 2θ=46°, whichis corresponding to a peak intensity of alumina other than α-alumina,from a diffraction spectrum measured under conditions of radiationsource: CuKα beam, 40 kV×20 mA, monochromator: graphite, by using apowder X-ray diffractometer:α-ratio=I _(25.6)/(I _(25.6) +I ₄₆)×100(%)  (i)(2) BET Specific Surface Area

It was measured by using specific surface area analyzer (trade name“FLOWSORB II 2300”, manufactured by SHIMADZU CORPORATION) with anitrogen adsorption method.

(3) Average Primary Particle Diameter

From a transmission electro micrograph of α-alumina powder, the maximumdiameter along constant direction of each primary particle of any 20 ormore particles was measured, and an average value of measured values wascalculated.

(4) Degree of Pulverizing

XRD spectrums of the seed crystal (a-alumina) before and afterpulverizing operations were measured by a X-ray diffractometer. The fullwidths at half maximum of a phase(116), i.e. HO(116)(before) andH(116)(after), were obtained from the XRD spectrums, followed bycalculation by the equation (ii)Degree of pulverizing=H(116)/H0(116)  (ii)

Example 1

[Preparation of Metal Compound(α-alumina)]

The aluminum hydroxide was obtained by hydrolyzing an aluminumisopropoxide, followed by pre-calcination to obtain a transition aluminain which the major crystal phase was θ phase and 3% by weight of a phasewas contained; the transition alumina was pulverized by a jet mill toobtain a powder having a bulk density of 0.21 g/cm³.

The obtained powder was calcined by a furnace filled with an air of −15°C. of dew point (partial pressure of water vapor: 165 Pa) in thefollowing conditions:

-   -   mode: continuous feeding and discharging,    -   average retention time: 3 hours,    -   maximum temperature: 1170° C.,        then α-alumina having full widths at half maximum of Ho₍₁₁₆₎,        BET specific surface area of 14 m³/g was obtained.        [Pulverization of α-alumina]

100 parts by weight of the α-alumina and 1 part by weight of a propyleneglycol as a pulverizing agent were charged into a vibration mill topulverize the α-alumina powder in the following conditions:

-   -   media: alumina beads having a diameter of 15 mm    -   retention time: 12 hours,        consequently, a seed crystal having a full widths at half        maximum of H₍₁₁₆₎ and BET specific surface area of 16.6 m³/g,        and average particle diameter of 0.1 μm was obtained. In this        example, a degree of pulverizing of H₍₁₁₆₎/Ho₍₁₁₆₎ is 1.1.        [Preparation of Seed Crystal Slurry]

In 80 parts by weight of 0.0001 mole/L aqueous aluminum nitratesolution, 20 parts by weight of the seed crystal dispersed to obtain aslurry. In a ball mill, the slurry and alumina beads having a diameterof 2 mm were charged into, and then agitated. The content of the ballmill was taken out to remove the alumina beads by filtration, then theseed crystal slurry was obtained.

[Mixing of Seed Crystal and Aluminum Salt]

375.13 g (1 mole) of aluminum nitrate nonahydrate(Al(NO₃)₃.9H₂O)(manufactured by Kansai Catalyst Co., Ltd., reagentgrade, appearance: powder) was dissolved in water to obtain 1000 cm³ ofan aluminum nitrate solution. 100 cm³ of the aluminum nitrate solutionwas added with 2.83 g of seed crystal described above (0.566 g in termsof Al₂O₃) to obtain a mixture. Then water was evaporated from themixture under conditions of 75° C. by using a pressure-reduced drier, toobtain a powder. The amount of the seed crystal in terms of Al₂O₃ was 10parts by weight per 100 parts by weight of the powder.

[Calcination]

2.8 g of the powder was charged into an alumina crucible, the aluminacrucible was placed in a box-type electric furnace at 25° C. The powderwas heated up to 850° C. at a rate of 300° C./h in air, and calcined at950° C. for 3 hours, to obtain an α-alumina powder. The α-alumina powderhas an α-ratio of 95%, BET specific surface area of 15.9 m²/g andaverage primary particle diameter of 96 nm.

1. A method for producing an α-alumina powder comprising steps of: (1)pulverizing a metal compound having a full width at half maximum (Ho) ofa main peak in XRD pattern to obtain a seed crystal having a full widthat half maximum (H) of the main peak in XRD pattern in the presence ofpulverizing agent, (2) mixing the obtained seed crystal with an aluminumsalt, (3) calcining the mixture, and wherein a ratio of H/Ho is 1.06 ormore.
 2. The method according to claim 1, wherein the metal compound isat least one selected from the group consisting of metal oxides andmetal hydroxides.
 3. The method according to claim 2, wherein the metalcompound is at least one selected from the group consisting of α-Al₂O₃,α-Fe₂O₃, α-Cr₂O₃ and diaspore.
 4. The method according to claim 1,wherein the ratio of H/Ho is 5 or less.
 5. The method according to claim1, wherein the pulverizing agent is at least one selected from the groupconsisting of alcohol, glycol, amine, fatty acid, metal alkoxide andcarbon material.
 6. The method according to claim 1, wherein thepulverizing agent is at least one selected from ethylene glycol,polyethylene glycol, propylene glycol, polypropylene glycol,1,3-butylene glycol and 1,4-butylene glycol.
 7. The method according toclaim 1, wherein the aluminum salt is inorganic aluminum salt.
 8. Themethod according to claim 7, wherein the inorganic aluminum salt isaluminum nitrate.
 9. The method according to claim 1, wherein theobtained seed crystal has a BET specific surface area of 12 m²/g ormore.
 10. The method according to claim 1, wherein amount of theobtained seed crystal is 1 part by weight based on 100 parts by weightof the total amount of the seed crystal and the aluminum salt.
 11. Themethod according to claim 1, Calcination is conducted at 600° C. ormore.